Common Errors in English - http://www.wsu.edu/~brians/errors/
This useful guide discusses close to 300 errors and goes on to a page about Non-errors, those usages people keep telling you are wrong but which are actually standard in English, another of commonly misspelled words, and a nice list of other good resources, including one on heteronyms and another on antagonyms.
Copy Editors - THE SLOT - http://www.theslot.com/
Here you'll find tips and thoughts on everything from word choice to punctuation, from quotations to capitalization, and everything in-between. The heart of the site is The Curmudgeon's Stylebook, an excellent reference source for writers.
Guide to Grammar and Writing - http://cctc.commnet.edu/grammar/index.htm
Extensive guide to effective use of the English language. Includes rules for punctuation, plurals and possessives, spelling, nouns and modifiers, words and phrases to avoid, capitalization, spelling and abbreviations, and a lot more. Quizzes and exercises included.